Wireless sensors, as well as micro-system integration of micromechanical devices with microelectronic circuits have become more and more important in modern microelectronic systems for industrial and military applications. These sensor systems have become much smaller, more sophisticated and less expensive. In order to power up these wireless sensors with long lasting wireless power sources, researchers have put their effort on energy harvesting devices instead of batteries.
MEMS (Micro-Electro-Mechanical Systems) energy harvesters have attracted much interest in the area of wireless sensors because of their simple structure and potential power density. The energy harvesting capabilities of piezoelectric energy harvesters highly depend on the vibration source, especially frequency matching. However, most vibration sources are frequency-varying or totally random in a certain frequency band. The geometric configuration and dimensions of an energy-harvesting device usually remain unchanged once the device has been implemented. Such a harvesting structure may become less effective in power generation when it operates in such a varying-frequency vibrating system. Hence, developing broadband EH has become an important problem for energy harvesting.
Current broadband EH strategies are: (1) resonance tuning, (2) multimodal tuning which includes multi-proof masses and EH arrays, and (3) frequency up-conversion. However, large size, unstable issues, and complex additional circuit problems limit the energy harvesting applications.
Further, conventional energy harvesters have a high Q (quality factor) and a sharp resonance peak, which may limit the frequencies the energy harvesters can harvest energy.